hwcontext_vaapi: Try the first render node as the default DRM device

hwcontext_vaapi: Try the first render node as the default DRM device

If no string argument is supplied when av_hwdevice_ctx_create() is
called to create a VAAPI device, we currently only try the default
X11 display (that is, $DISPLAY) to find a device, and will therefore
fail in the absence of an X server to connect to.  Change the logic
to also look for a device via the first DRM render node (that is,
"/dev/dri/renderD128"), which is probably the right thing to use in
most simple configurations which only have one DRM device.